Helical Slow-Wave Circuit Assembly

Abstract: A helical slow-wave circuit assembly includes a helix (101), P-BN pillars (102-104), a metal cylinder (111), artificial diamond films (105-107), and intermediate films (108-110). The helix (101) constitutes a slow-wave circuit of the electromagnetic wave with respect to an electron flow and generates an electromagnetic wave that travels as a current flows. The P-BN pillars (102-104) are disposed equidistantly around the helix (101) in a direction along which the electromagnetic wave travels, and contains at least nitrogen. The metal cylinder (111) supports the helix (101) therein through the P-BN pillars (102-104). The artificial diamond films (105-107) are formed on outer circumferential surfaces of the P-BN pillars (102-104). The intermediate layers (108-110) are formed between the P-BN pillars (102-104) and the artificial diamond films (105-107) and prevents diffusion of nitrogen from the P-BN pillars (102-104) to the diamond films (105-107).

Description: Background of the Invention The present invention relates to a helical slow-wave circuit assembly and, more particularly, to a helical slow-wave circuit assembly utilized in, e.g., a traveling-wave tube and a backward-wave tube. Electron beams pass through a helical slow-wave circuit in a traveling-wave tube or a backward-wave tube as they partly come close to the circuit. Thus, the helical slow-wave circuit is heated by heat generated when the electron beams partly collide against the helical slow-wave circuit, or by heat generated by resistance loss of an RF power transmitted through the helical slow-wave circuit. Due to this heating function, a helical slow-wave circuit having a comparatively small heat capacity reaches a rather high temperature. This leads to an increase in RF power loss and an increase in gas emitted from the slow-wave circuit, leading to a decrease in output of the traveling- or backward-wave tube, degradation in beam transmittance, an increase in noise, and the like, as well as a decrease in service life. In recent years, demands for a higher frequency and a higher output of a traveling-wave tube are increasing. In a helical slow-wave tube used for these purposes, the dielectric constant and the heat conductivity of dielectric pillars are important factors in addition to the heat resistance of the helix and the means for emitting heat from the helix. In a conventional helical slow-wave circuit assembly, a helix is formed by using a round wire or a tape, and a plurality of cylindrical or prismatic dielectric pillars are disposed around the helix. This structure is housed in a metal cylinder, and the helix and the dielectric pillars are clamped and fixed by using an appropriate means. An example of this assembly will be described with reference to Figs. 4A and 4B. Fig. 4A is a sectional view of a conventional helical slow-wave circuit assembly, and Fig. 4B is a partial enlarged sectional view of a portion in the vicinity of a boron nitride (to be described as P-BN hereinafter) pillar of this assembly. As shown in Figs. 4A and 4B, a helix 1 constituting the slow-wave circuit is made by forming tungsten or molybdenum, which is not easily softened or deformed by collision of electron beams and which has a comparatively high melting point, into a wire or a tape, and coiling it in a helical manner. Three prismatic P-BN pillars 2 to 4 are disposed around the helix 1 at angular intervals of 120°, and a metal cylinder 11 is provided to surround the P-BN pillars 2 to 4. The P-BN pillars 2 to 4 have a multilayered structure. A direction of each P-BN pillar parallel to the layers is called a direction a , and a direction thereof perpendicular to the layers is called a direction c . Generally, in P-BN, the directions a and c have largely different physical and mechanical characteristics, and characteristics in the direction a are superior to those in the direction c . Therefore, the P-BN pillars 2 to 4 are provided such that the directions a and c become respectively perpendicular and parallel to the contact surfaces of the helix 1 with the P-BN pillars 2 to 4. In order to prevent concentration of the mechanical stress which occurs upon insertion of the P-BN pillars 2 to 4 into the metal cylinder 11, the outer and inner circumferential surfaces of the respective P-BN pillars 2 to 4 are formed in accordance with a radius R of curvature of the metal cylinder 11 and the helix 1. Furthermore, conventionally, artificial diamond films 5 to 7 having a thickness of several µm are formed on the outer circumferential surfaces of the P-BN pillars 2 to 4, respectively, in accordance with chemical vapor deposition (to be referred to as CVD hereinafter), ion plating (to be referred to as IP hereinafter), or the like in order to increase the heat conductivity and mechanical strength. The mechanical strength of the P-BN pillars 2 to 4 need be increased due to the following reason. More specifically, when the helix 1 and the P-BN pillars 2 to 4 are clamped by the metal cylinder 11, shearing stresses act on the helix 1 at portions corresponding to the central portions of the P-BN pillars and on the metal cylinder 11 corresponding to the two ends of each P-BN pillar. As described above, however, because of the characteristics and the manufacture of the P-BN pillars, the P-BN pillars 2 to 4, the helix 1, and the metal cylinder 11 contact with each other through the surfaces in the axis of the direction c . When the helix 1 and the P-BN pillars 2 to 4 are clamped by the metal cylinder 11 in accordance with squeezing to be described later, a shearing stress acts on the P-BN pillars 2 to 4, causing many cracks. This cracking adversely affects the RF characteristics of the traveling-wave tube and decreases the output and gain of the traveling-wave tube. Furthermore, when the cracks formed in the P-BN pillars 2 to 4 during clamping are subjected to thermal hysteresis and progress due to the operation of the traveling-wave tube, the traveling-wave tube may cause an operation error, which is a critical defect. The artificial diamond films 5 to 7 are formed on the outer circumferential surfaces of the P-BN pillars 2 to 4 in order to eliminate these drawbacks. Regarding the metal cylinder 11, since a means for applying a magnetic field for focusing the electron beams traveling in the helix 1 is to be arranged around the metal cylinder 11, mainly a stainless-steel tube, and recently, a tube constituted by layers of iron and a copper alloy and serving also as a vacuum envelope along with downsizing, are used as the metal cylinder 11. To insert the helix 1 and the P-BN pillars 2 to 4 having the artificial diamond films 5 to 7 into the metal cylinder 11, for example, the metal cylinder 11 is heated to utilize thermal expansion, or a pressure is applied to the outer surface of the metal cylinder 11 in three directions to utilize mechanical deformation (squeezing). After insertion, heat or pressure is removed from the metal cylinder 11, so that the helix 1 and the P-BN pillars 2 to 4 are fixed and clamped, thereby completing a helical slow-wave circuit assembly. In the conventional helical slow-wave circuit assembly described above, however, the P-BN pillars 2 to 4 are heated when the artificial diamond films 5 to 7 are formed on the P-BN pillars 2 to 4 in accordance with CVD or IP and when the traveling- or backward-wave tube operates, so that nitrogen (N) in the P-BN pillars 2 to 4 is diffused to the artificial diamond films 5 to 7, thereby decreasing the electrical resistance of the artificial diamond films 5 to 7. More specifically, diffusion of nitrogen (N) caused by heating the P-BN pillars 2 to 4 decreases the electrical resistance (resistivity) of the artificial diamond films 5 to 7 from 10 11 Ω·cm to as low as 10 5 to 10 6 Ω·cm. When the electrical resistance of the surfaces of the P-BN pillars 2 to 4 serving as dielectric pillars is decreased in this manner, loss of the RF wave amplified while being transmitted through the helix 1 becomes considerably large, thereby extremely decreasing the output of the traveling- or backward-wave tube, which is a critical drawback. A helical slow-wave circuit assembly according to the preamble of claim 1 is disclosed in EP-A-0507195. Summary of the Invention It is an object of the present invention to provide a high-reliability helical slow-wave circuit assembly in which a decrease in output of a traveling-or backward-wave tube caused by RF loss is prevented. In order to achieve the above object, according to the present invention, there is provided a helical slow-wave circuit assembly comprising a helix, constituting a slow-wave circuit of the electromagnetic wave with respect to an electron flow, for generating an electromagnetic wave that travels as a current flows, a plurality of dielectric pillars disposed equidistantly around the helix in a direction along which the electromagnetic wave travels and containing at least nitrogen, a metal cylinder for supporting the helix therein through the dielectric pillars, artificial diamond films formed on outer circumferential surfaces of the dielectric pillars, and intermediate layers, formed between the dielectric pillars and the artificial diamond films, for preventing diffusion of nitrogen from the dielectric pillars to the artificial diamond films. Particular embodiments of the invention are set out in the dependent claims. Brief Description of the Drawings Fig. 1A is a sectional view showing a helical slow-wave circuit assembly according to the first embodiment of the present invention, and Fig. 1B is a partially cutaway perspective view of the same; Fig. 2 is a partial enlarged sectional view of a portion in the vicinity of a P-BN pillar shown in Figs. 1A and 1B; Fig. 3A is a sectional view showing the second embodiment of the present invention, and Fig. 3B is a partial enlarged sectional view showing a portion in the vicinity of a P-BN pillar of the same; and Fig. 4A is a sectional view of a conventional helical slow-wave circuit assembly, and Fig. 4B is a partial enlarged sectional view of a portion in the vicinity of a P-BN pillar of the same. Description of the Preferred Embodiments The preferred embodiments of the present invention will be described with reference to the accompanying drawings. Figs. 1A and 1B show the first embodiment of the present invention, and Fig. 2 shows a portion in the vicinity of a P-BN pillar shown in Figs. 1A and 1B. Referring to Figs. 1A, 1B, and 2, reference numeral 101 denotes a helix constituting a slow-wave circuit of an electromagnetic wave. The helix 101 almost equalizes the traveling speed of the electromagnetic wave generated by a current flowing in the helix 101 with the speed of an electron beam A emitted from an electron gun (not shown) and passing through the helix 101. Reference numerals 102 to 104 denote three prismatic P-BN pillars disposed around the helix 101 at angular intervals of 120° in the traveling direction of the electromagnetic wave. Reference numeral 111 denotes a metal cylinder provided around the helix 101 through the P-BN pillars 102 to 104 as spacers. The P-BN pillars 102 to 104 are constituted by multilayered structures each having a direction a parallel to the layers and a direction c perpendicular to the layers. The P-BN pillars 102 to 104 are disposed such that the directions a and c are respectively perpendicular and parallel to the contact surfaces with the helix 101. The outer and inner circumferential end faces of the P-BN pillars 102 to 104 are formed in accordance with a radius R of curvature of the metal cylinder 111 and the helix 101 in order to prevent concentration of the mechanical stress which occurs upon insertion into the metal cylinder 111. Artificial diamond films 105 to 107 are formed on the outer circumferential surfaces of the P-BN pillars 102 to 104 in order to increase the heat conductivity and the mechanical strength in the same manner as in the conventional helical slow-wave circuit assembly. TiC layers 108 to 110 which do not easily react with the P-BN pillars 102 to 104 and the artificial diamond films 105 to 107 upon heat treatment are formed between the P-BN pillars 102 to 104 and the artificial diamond films 105 to 107 as intermediate layers. A method of manufacturing the helical slow-wave circuit assembly having the above arrangement will be described. The P-BN pillars 102 to 104 have a height of 1 mm, a width of 0.5 mm, and a length of 100 mm, and their two surfaces in the widthwise direction are arcuatedly formed. The titanium carbide (to be referred to as TiC hereinafter) layers 108 to 110 are formed on the surfaces of the P-BN pillars 102 to 104 to a thickness of 1 to 2 µm in accordance with plasma CVD, and subsequently the artificial diamond films 105 to 107 are formed on the TiC layers 108 to 110 to a thickness of about 100 µm in accordance with plasma CVD. The helix 101 is made of tungsten, formed into a tape having a width of 1.5 mm and a thickness of 1 mm, and coiled to have an inner diameter of 2 mm. The P-BN pillars 102 to 104, having the TiC layers 108 to 110 and the artificial diamond films 105 to 107 formed thereon, are disposed around the helix 101 at angular intervals of 120°. The helix 101 and the P-BN pillars 102 to 104 are housed in the 0.4-mm thick 120-mm length metal cylinder 111 made of stainless steel, thereby completing a helical slow-wave circuit assembly. In this case, the metal cylinder 111 is deformed by applying a pressure to its outer circumferential surface in three directions in accordance with squeezing. The helix 101 and the P-BN pillars 102 to 104 are inserted in the metal cylinder 111 by using an appropriate jig (not shown). Then, the pressure applied to the metal cylinder 111 is removed, so that the helix 101 and the P-BN pillars 102 to 104 are clamped by the restoring force of the metal cylinder 111. According to this method, diffusion of nitrogen from the P-BN pillars 102 to 104 to the artificial diamond films 105 to 107 can be prevented in the presence of the TiC layers 108 to 110 respectively formed between the P-BN pillars 102 to 104 and the artificial diamond films 105 to 107. Figs. 3A shows the second embodiment of the present invention, and Fig. 3B shows a portion in the vicinity of a P-BN pillar of the same. According to the second embodiment, as shown in Figs. 3A and 3B, silicon carbide (to be referred to as SiC hereinafter) layers 112 to 114 are respectively formed between P-BN pillars 102 to 104 and artificial diamond films 105 to 107 in accordance with IP. The second embodiment is the same as the first embodiment except that the SiC layers 112 to 114 are respectively formed between the P-BN pillars 102 to 104 and the artificial diamond films 105 to 107 in accordance with IP. It is known that SiC reacts less with the P-BN pillars and the diamond films in the same manner as TiC, so that the same effect as that of the first embodiment can be obtained. As has been described above, according to the present invention, diffusion of nitrogen (N) from the P-BN pillars to the artificial diamond films can be prevented by providing, between nitrogen-containing dielectric pillars made of P-BN or the like and artificial diamond films, intermediate layers made of TiC or SiC which does not easily react with the dielectric pillars and the artificial diamond films upon heating. Then, the electric resistance of the diamond films is increased from a conventional value of 10 5 to 10 6 Ω·cm up to 10 11 Ω·cm which is a value diamond should originally have, thereby preventing a decrease in electric resistance. As a result, loss in RF wave transmitted through the helix during operation of the traveling- or backward-wave tube becomes about 1/2 that the conventional value, so that a high-output, high-reliability slow-wave circuit assembly of a traveling-wave tube or the like can be obtained.

Claims: A helical slow-wave circuit assembly comprising: a helix (101), constituting a slow-wave circuit of the electromagnetic wave with respect to an electron flow, for generating an electromagnetic wave that travels as a current flows; a plurality of dielectric pillars (102 - 104) disposed equidistantly around said helix in a direction along which the electromagnetic wave travels and containing at least nitrogen; a metal cylinder (111) for supporting said helix therein through said dielectric pillars; artificial diamond films (105 - 107) formed on outer circumferential surfaces of said dielectric pillars; characterised in that intermediate layers (108 - 110; 112 - 114), formed between said dielectric pillars and said artificial diamond films, prevent diffusion of nitrogen from said dielectric pillars to said artificial diamond films. An assembly according to claim 1, wherein said intermediate layers are made of titanium carbide. An assembly according to claim 1, wherein said intermediate layers are made of silicon carbide. An assembly according to claim 1, wherein said dielectric pillars are made of boron nitride. An assembly according to claim 1, wherein said intermediate layers are ion plated layers. An assembly according to claim 1, wherein said intermediate layers are chemical vapor deposited layers.

Inventors: NISHIDA KAZUHISA, NISHIDA, KAZUHISA, NISHIDA, KAZUHISA, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1994
Main classification: H01J23
IPC codes: H01J23, H01J9